In general, control of the air-fuel ratio in an engine, whether gasoline or alcohol, is carried out such that, first of all, a basic control factor such as the fuel injection quantity (or injection time) of a fuel injector is adjusted according to engine driving conditions such as intake air amount per engine revolution and/or intake pressure in an intake passage. The basic fuel injection quantity is then modified by multiplying various air-fuel ratio correction coefficients, such as an engine temperature correction coefficient and/or a feedback control correction coefficient, to finally obtain a target fuel injection quantity (or injection time), thereby controlling the actual air-fuel ratio to conform to the theoretical air-fuel ratio at any given time.
Moreover, the feedback coefficient is decreased when the air-fuel ratio, judged by the oxygen density in the exhaust gas detected by an oxygen (O.sub.2) sensor, is smaller than the theoretical air-fuel ratio of 14.7 (i.e., lean). Conversely, the feedback coefficient is increased when the air-fuel ratio is larger than the theoretical air-fuel ratio (i.e., rich).
The O.sub.2 sensor includes a zirconia element which generates a certain amount of voltage when oxygen ions pass through it. The O.sub.2 sensor is usually constructed to introduce an atmospheric reference gas inside the zirconia element, and exhaust gas outside the zirconia element. Therefore, when the oxygen density of the exhaust gas is high, that is, when its air-fuel ratio is larger than the theoretical air-fuel ratio (lean), the voltage generated in the zirconia element becomes small. On the other hand, when the oxygen density of the exhaust gas is low (rich), the voltage generated in the zirconia element becomes large.
However, it should be noted that, if an engine uses fuel containing an alcohol component, the amount of hydrogen (H.sub.2) in the combustion gas becomes larger compared with the H.sub.2 amount in a combustion gas generated from gasoline. Since a H.sub.2 molecule is smaller than an O.sub.2 molecule, the H.sub.2 molecule diffuses with greater speed than the O.sub.2 molecule and thus the H.sub.2 molecule enters into the zirconia element more easily than the O.sub.2 molecule. Therefore, the O.sub.2 sensor mistakenly judges the oxygen density as if it were lower than the actual density, due to this diffusion disparity between H.sub.2 and O.sub.2. The result is the so-called lean shift phenomenon, where the output characteristic of the O.sub.2 sensor shifts toward the lean side, as shown by the dotted line in FIG. 6.
Accordingly, if the feedback control is carried out under the lean shift condition, the air-fuel ratio is controlled to be much leaner than required for operation of a rhodium catalytic converter as an exhaust gas purification device. A resulting problem is that the purification efficiency of NOx deteriorates.
Japanese laid-open patent application No. 01/244,133 discloses a fuel injection control apparatus in which the lean shift phenomenon of the O.sub.2 sensor is modified by correcting the air-fuel ratio to be richer as an alcohol density, by which the amount of H.sub.2 in the combustion gas increases, becomes large. The lean shift phenomenon of the O.sub.2 sensor due to the alcohol density of the fuel can be effectively suppressed with this apparatus.
However, even though the lean shift phenomenon of the O.sub.2 sensor is modified based on the alcohol density of the fuel, the air-fuel ratio is still controlled to be lean, and thus it is feared that the efficiency of the rhodium catalytic converter is impaired.